Abstract. We report on the first multi-year springtime measurements of PAN in the free
troposphere over the US Pacific Northwest. The measurements were made at
the summit of Mount Bachelor (43.979° N, 121.687° W; 2.7 km a.s.l.) by
gas chromatography with electron capture detector during spring 2008, 2009
and 2010. This dataset provides an observational estimate of the
month-to-month and springtime interannual variability of PAN mixing ratios
in this region. Springtime seasonal mean (1 April–20 May) PAN mixing
ratios at Mount Bachelor varied from 100 pptv to 152 pptv. The standard
deviation of the three seasonal means was 28 pptv, 21 % of the springtime
mean. We summarize the interannual variability in three factors expected
to drive PAN variability: biomass burning, transport efficiency over the
central and eastern Pacific, and transport temperature.
Zhang et al. (2008) used the
GEOS-Chem global chemical transport model to show that rising Asian NOx
emissions from 2000 to 2006 resulted in a relatively larger positive trend
in PAN than O3 over western North America. However the model results
only considered monotonic changes in Asian emissions, whereas other factors,
such as biomass burning, isoprene emissions or climate change can induce
greater variability in the atmospheric concentrations and thus extend the
time needed for trend detection. We combined the observed variability in PAN
and O3 at Mount Bachelor with a range of possible future trends in
these species to determine the observational requirements to detect such
trends. Though the relative increase in PAN is expected to be larger than
that of O3, PAN is more variable. If PAN mixing ratios are currently
increasing at a rate of 4 % per year due to rising Asian emissions, we
would detect a trend with 13 years of measurements at a site like Mount
Bachelor. If the corresponding trend in O3 is 1 % per year, the
trends in O3 and PAN would be detected on approximately the same
timescale.